The present invention relates in general to trailing arm suspension systems and in particular to improvements in trailing arm air suspensions incorporating a spring steel Z-beam or step spring pivotally connected to a hanger.
Various suspension systems have been designed and developed incorporating an air spring. Some trailing arm air suspensions are provided with rigid axle attachment. The trailing arm rotates about a point or a zone near the front of the trailing arm. In some cases this point is a pivot bush contained in an eye formed at the front end of the trailing arm which allows rotation of a trailing arm but prevents vertical or horizontal movement. In other cases the pivot zone is at a slipper pad which prevents vertical movement of the trailing arm, combined with a link (torque rod or track rod) which prevents horizontal (push/pull) movement. Normal movement of this type of suspension therefore means that the main support member rotates about its forward end. Vertical movement at its rear end is controlled by the compressive forces of the air spring and the damping effects of shock absorbers. At some position in between the front and rear end of the trailing arm the truck drive axle is rigidly attached to the trailing arm. This is usually achieved by clamping through U bolts and positioning by a system of spring seats and axle caps which locate the axle at a particular angle at normal ride height of the suspension. Being located thus, the axle has vertical movement capability, relative to the truck chassis, controlled by the air spring and shock absorbers. Horizontal restraint, front to rear (push/pull loads) is provided via rigid attachment of the axle to the trailing arm and lateral restraint of the axles provided by a transverse torque rod arrangement which is located across the vehicle between the axle housing and the chassis. This transverse rod arrangement is sometimes known as a Panhard rod. These types of suspensions, whilst generally satisfactory, have a disadvantage in that, as well as the axle moving vertically, which is a suspension requirement, it also rotates together with the trailing arms. This axle rotation has two negative effects as follows: (1) the axle angle changes throughout the normal range of suspension movement and this is detrimental to the universal joints used to transmit driving torque to the angle; (2) the suspension is reactive to driving and braking torques which can lead to unwanted suspension movement, sometimes referred to as xe2x80x9cwind-upxe2x80x9d, during application of normal drive line driving and braking torque. Attempts to fit an upper longitudinal torque rod to suspensions of the above type fail because, while the purpose of a longitudinal torque rod is to prevent axle rotation, in suspensions of this type the axle must rotate together with the trailing arm as it is rigidly attached to the trailing arm.
Attempts have been made to construct suspensions in which the axle pivots on the trailing arm. In one type of suspension the axle pivots on the trailing arm through a bush or bearing located below the axle. Suspensions of this type are shown in U.S. Pat. No 4,309,045 and they must have an upper torque rod linking a bracket on the top of the axle to the truck chassis. This gives a parallelogram linkage or a 4-bar chain type of mechanism. This means that the axle can move vertically without rotation and that the driving and braking torques have relatively little influence on the suspension movement. However, this design generally increases the mass of the suspension significantly and, because of the below axle connection to the trailing arm, vehicle ground clearance is reduced, which is a disadvantage in off-road applications.
The present invention aims to alleviate the above-mentioned problems.
According to one aspect of the present invention there is provided a suspension system for installation between a chassis having side rail members and cross frame members and an axle of a vehicle, said suspension system including a step spring having a forward generally horizontal section having a leading end, a rearward generally horizontal section having a trailing end, a generally vertical section between and contiguous with the forward and rearward sections and displacing the rearward section below the forward section, said leading end of said step spring being pivotally attached to said chassis, spring means extending between said chassis and said trailing end of said step spring and said axle being pivotally secured to a mounting means on said forward generally horizontal section, said pivot being above said axle.
Preferably said leading end is pivotally attached to a hanger on said chassis and a torque rod or other member links said chassis to said axle. In a preferred embodiment said spring means includes an air spring. Preferably said torque rod or other member is coupled to one of said side rail members. In a practical embodiment a further torque rod links said hanger to said axle.